CN110249115B

CN110249115B - Pump actuator with stamped aligned anti-rotation features

Info

Publication number: CN110249115B
Application number: CN201880007976.8A
Authority: CN
Inventors: 道格拉斯·雷·科内特; 亚当·格雷格·保罗
Original assignee: Eaton Intelligent Power Ltd
Current assignee: Eaton Intelligent Power Ltd
Priority date: 2017-01-27
Filing date: 2018-01-23
Publication date: 2023-09-15
Anticipated expiration: 2038-01-23
Also published as: US20230160385A1; DE112018000288T5; WO2018140403A1; US20190368485A1; US11578717B2; CN110249115A

Abstract

The present disclosure relates to a tappet, which may be a fuel pump actuator, including a body that is a shell-hardened ferrous metal abutment. The body has a cylinder conforming bore running surface and an anti-rotation guide feature that has been made by stamping to protrude outwardly from the bore running surface. The cam follower is mounted to the body. The body has been hardened by the ferritic nitrocarburizing shell, but has not been distorted by the heat treatment, and has a malleable interior. The cross-web for the lifter may be fully stiffened. The shaft hole for the cam follower may be formed by punching. The tappet is durable and provides excellent alignment of the roller with the cam, which reduces friction and noise.

Description

Pump actuator with stamped aligned anti-rotation features

Technical Field

The present disclosure relates to lifters for internal combustion engines, particularly pump actuators for high-pressure fuel pumps.

Background

The tappet converts the rotational movement of the cam into a reciprocating movement. The tappet body generally includes a cylinder conforming bore running surface that guides the tappet as it reciprocates within the bore. The roller may be mounted at the drive input of the tappet body to follow the cam of the drive tappet. High pressure fuel pump actuation is one of the more demanding tappet applications. Pump actuator lifters typically require hardened ferrous metal to meet operational life requirements.

The alignment between the roller and the cam is critical to keep friction and noise within acceptable limits. The tappet may have an anti-rotation guide feature to maintain the roller axis aligned with the cam axis in a plane. The roller axis may be maintained perpendicular to the bore axis and parallel to the cam axis depending on the mounting of the roller to the tappet body.

Disclosure of Invention

One aspect of the present teachings is a tappet including a body that is a shell-hardened ferrous metal abutment. The body has a cylinder conforming bore running surface and an anti-rotation guide feature that has been made by stamping to protrude outwardly from the bore running surface. The cam follower is mounted to the body. The tappet according to the present teachings provides excellent alignment of the roller with the cam, which reduces friction and noise.

Another aspect of the present teachings is a tappet formed by a method comprising: forming a body from a ferrous metal having a surface that includes a cylinder conforming bore running surface, stamping the body to form an anti-rotation guide feature protruding outward from the bore running surface, hardening the body housing by adding nitrogen to the ferrous metal at a subcritical temperature, and attaching a cam follower to the body.

Another aspect of the present teachings is a method of manufacturing a tappet. The method includes forming a ferrous metal sheet to provide a cylinder bore running surface, forming an anti-rotation guide feature protruding outward from the cylinder bore running surface by stamping, hardening a body housing by adding nitrogen to the ferrous metal while maintaining the ferrous metal in a ferritic state, and mounting a cam follower at one end of the body.

The alignment of the rollers is largely determined by the geometric relationship between the roller mounting, bore running surface and anti-rotation guide. In the tappet according to the present teachings, these relationships may be controlled by a stamping method. The stamping forms one or more anti-rotation guide features on the body that relate to the relative position of the rollers. In some of these teachings, the body includes two parallel planar surfaces formed by stamping and adjacent to the drive input end of the body. Shaft holes may be formed in these flat surfaces, and axial support pins for cam followers may be installed through those shaft holes. The orientation of those planar surfaces relative to the anti-rotation guide features aids in roller alignment. In some of these teachings, the shaft bore for the cam follower is formed by stamping. The stamping method to form the shaft aperture may include piercing and shaving. Forming the shaft holes by stamping improves roll alignment.

The cylinder conforming bore running surface is operable to engage the first cylinder bore to guide translation of the tappet within the bore. The axis of the bore running surface coincides with the bore axis. The cam is arranged with its contact surface perpendicular to the bore. In some of these teachings, the anti-rotation guide feature is operable to engage a second cylinder bore having a smaller diameter than and intersecting the first cylinder bore. In this configuration, the anti-rotation guide feature limits rotation of the tappet within the first cylinder bore.

According to some of the present teachings, the body is not subjected to any hardening method that heats the metal above a critical temperature. The critical temperature is the temperature at which the metal transforms from the ferrite phase to the austenite phase. For example, the body is not subjected to carbonitriding, which is a conventional case hardening method. Carbonitriding involves heating the metal above a critical temperature. If the body is subjected to carbonitriding prior to stamping, the metal will not have sufficient ductility for the stamping process. If the body is subjected to carbonitriding after stamping, the cylinder conforming bore running surface will be distorted and the anti-rotation guide features will interfere with machining to restore roundness to the bore running surface.

The body of the prior art tappet is subjected to carbonitriding. The hardening method results in shape distortion. The outer surface of the body is returned to the cylinder conforming shape by a method such as OD grinding, which removes metal from the surface. However, it was found that carbonitriding and OD grinding can alter the geometric relationship between the roll mounting and bore running surfaces. In the present teachings, these methods can be avoided. In some of the present teachings, the body lacks the type of distortion that would result from a hardening process involving heating the body above a critical temperature.

In some of the present teachings, the bore running surface does not bear any evidence of operation that has helped determine its outer diameter and has not yet been applied to the surface of the anti-rotation guide. In some of the present teachings, the final outer diameter of the cylinder conforming bore running surface is produced without any grinding, milling or milling affecting the outer diameter. The outer diameter may be largely determined prior to stamping, although the shell hardening may have a measurable effect on the outer diameter. In some of the present teachings, the body is formed from sheet metal by deep drawing. In some of the present teachings, the outer diameter of the body is determined by a method consisting essentially of deep drawing, stamping, and shell hardening. In some of the present teachings, the metal that provides the bore running surface is present at the surface of the body prior to stamping. The body is shell hardened, but in accordance with some of the present teachings, the body has a relatively malleable interior.

In some aspects of the present teachings, the tappet is a pump actuator. In some of these teachings, the tappet is a high-pressure fuel pump actuator. Pump actuator applications require high fatigue resistance. In the present teachings, the body shell is hardened by a method of adding nitrogen to the ferrous metal while maintaining the ferrous metal in a ferritic state. In some of these teachings, the shell hardening method is ferritic nitrocarburizing. The case hardening method is a method of modifying metal near the surface of the component to provide a hardened case.

The cross beam may be mounted within the body. In some of these teachings, the beam is ferrous metal that hardens over its full thickness, while the body has a malleable interior. Hardening the beam over its full thickness includes heating the beam to a temperature at which the ferrous metal enters the austenitic phase. In some of these teachings, the beam is mounted within the body by a method that includes crimping to secure the beam within the body.

Since the anti-rotation guide feature is formed by stamping metal that also provides the bore running surface, the anti-rotation guide feature abuts the bore running surface. In some of these teachings, the anti-rotation guide feature has a length that extends along an axis of the cylinder bore running surface, and the anti-rotation guide feature contacts the cylinder bore running surface along two opposite sides of the anti-rotation guide feature, both of which extend along the length. In some of these teachings, the interface between the anti-rotation guide feature and the cylinder conforming bore running surface forms a perimeter around the anti-rotation guide feature. This means that the anti-rotation guide is continuous with the bore running surface on all sides.

In some of these teachings, the body includes two parallel planar surfaces at its drive input, an axle bore is formed in each of the two planar surfaces, an axial support pin for the cam follower is mounted through the axle bore, and the body further includes two substantially planar additional surfaces. The two additional surfaces are in the transition region between the cylinder conforming bore running surface and the two parallel flat surfaces. The additional surface is adjacent to the parallel planar surface at an end of the parallel planar surface distal to the drive input end of the body. In some of these teachings, the additional surface is inclined relative to the axis of the cylinder bore running surface and the inclination angle is in the range of 15 degrees to 75 degrees. Tilting those surfaces in this manner reduces the weight of the tappet while maintaining or increasing its fatigue resistance.

The primary purpose of the summary is to present some concepts of the inventors in a simplified form to facilitate an understanding of the more detailed description that follows. This summary is not an extensive overview of each and every concept of the inventors, or every combination of the inventors' concepts that may be considered to be "inventive". Other concepts of the present inventors will be readily apparent to those of ordinary skill in the art from the following detailed description, taken in conjunction with the accompanying drawings. The details disclosed herein may be summarized, scaled down, and combined in various ways with the inventors' invention as claimed by the final statement of the claims that follow.

Drawings

Fig. 1 is a perspective view of a tappet according to some aspects of the present teachings.

Fig. 2 is a perspective view of a body of the tappet of fig. 1 prior to assembly.

Fig. 3 is a perspective view of a cross beam of the tappet of fig. 1.

Fig. 4 is a perspective view of the body of fig. 2 and the cross beam of fig. 3 after assembly.

Fig. 5 is a sketch showing the measurement of perpendicularity.

Fig. 6 is a cross-section taken on line 6-6 of fig. 4.

Fig. 7 is an illustration of the lifter of fig. 1 installed in an engine to operate as a fuel pump actuator in accordance with aspects of the present teachings.

Fig. 8 is a cross-section taken on line 8-8 of fig. 4, but showing a tappet as installed in the engine of fig. 7.

Fig. 9 is a flow chart of a method in accordance with some aspects of the present teachings.

Detailed Description

Fig. 1 is a perspective view of a lifter 100, which is an example of some according to the present teachings. The tappet 100 includes a body 101, a cross beam 121 (not visible in fig. 1), and a cam follower 131. Fig. 2 is a perspective view of the main body 101. Fig. 3 is a perspective view of the cross beam 121. As shown in fig. 4 and 6, the cross member 121 is installed in the main body 101. Fig. 4 is a perspective view of the main body 101 and the cross beam 121, and fig. 6 is a sectional view corresponding to fig. 4.

The cam follower 131 includes an axial support pin 133, a bearing 137, and a roller 135. The roller 135 is mounted on the axial support pin 133 by a bearing 137. The cam follower 131 is mounted to the body 101 adjacent the drive input 103. The cross beam 121 may rest on a flange 125 formed on the inside of the body 101. The cross beam 121 may be secured against the flange 125 by a dimple 123, which may be formed in the body 101 by crimping. Both the body 101 and the cross beam 121 are formed of ferrous metal (which is steel). The cross beam 121 is hardened throughout its thickness, while the body 101 is only shell hardened and has a malleable interior. The hardened material (which includes the outer shell of the body 101 and the interior of the cross beam 121) has a hardness of greater than 500 HV. The malleable material has a hardness of less than 500 HV. 500HV is the number of Vickers pyramids based on the Vickers hardness test.

The shell hardening may harden only the metal within 100 microns of the surface. In some of these teachings, hardening is limited to within 50 microns of the surface. In some of these teachings, hardening is limited to within 30 microns of the surface. The hardened layer may have a thickness between about 10 microns and 15 microns. The distribution of hardening can be determined by forming portions and using hardness traces.

The body 101 has a cylinder conforming bore running surface 109. The surface 109 is the outer surface of the body 101. It is cylinder-shaped in that it follows the shape of a cylinder having an axis 151. Although the surface 109 conforms to the shape of the cylinder, it need not form any complete cylinder in itself. Surface 109 is a bore running surface in that when installed in a mating bore it is operable to guide translation of tappet 100 and will limit wobble within the bore.

The body 101 is a ferrous metal abutment that includes an anti-rotation guide feature 115. The body 101 has been stamped such that the anti-rotation guide features 115 are formed as outward protrusions from the cylinder conforming bore running surface 109. The formation of the anti-rotation guide feature 115 by stamping is evident by its continuity with the metal forming the bore running surface 109. The anti-rotation guide feature 115 has a length 153 that extends parallel to the axis 151 and contacts the bore running surface 109 on both sides 117 that extend along the length 115. Preferably, the anti-rotation guide feature 115 contacts the bore running surface 109 through a substantial portion of the length 115. More preferably, the anti-rotation guide feature 115 contacts the bore running surface 109 through its entire length 115. Still more preferably, the anti-rotation guide feature 115 contacts the bore running surface 109 around its entire circumference, as shown in the figure for the tappet 100. These continuity features may be achieved by forming the anti-rotation guide features 115 in a stamping process.

The body 101 comprises two parallel planar surfaces 105 adjacent to the drive input 103. The cam follower 131 includes an axial support pin 133 mounted to the body 101 through an axial bore 111 formed in the surface 105. The surface 105 is stamped into the body 101. The orientation of the cam follower 131 relative to the anti-rotation guide feature 115 is related to the orientation of the surface 105 relative to the anti-rotation guide feature 115. The orientation of the cam follower 131 relative to the anti-rotation guide feature 115 is improved by the stamping of both the surface 105 and the anti-rotation guide feature 115. In addition, the shaft bores 111 are also formed by stamping, which further improves their orientation relative to the anti-rotation guide features 115 and relative to the bore running surface 109.

A high degree of perpendicularity is achieved between the bore running surface 109 and the cam follower 131. Fig. 5 shows the measurement of perpendicularity. Perpendicularity is measured as an end-to-end variation of the distance of the roller 135 from a plane 163 perpendicular to the axis 151 of the bore running surface 109. The change is the difference between distance 165 and distance 167. The roller 135 typically has a length in the range of about 5mm to about 20mm, with 11mm being the length in this example. For rolls of this size, it is desirable to maintain perpendicularity below 45 microns. The present teachings allow for verticality below 30 microns to be achieved. For lifter 100, the verticality is about 20 microns. In order to correlate these verticals to other sized rolls, they can be normalized to 11mm roll length to yield dimensionless verticals of 0.0041, 0.0027 and 0.0018.

Verticality is partly the result of not yet completing an operation on the body 101. The bore running surface 109 has not been subjected to a heat treatment method that would distort its shape. The bore running surface 109 has not been subjected to an OD grinding or any other grinding, milling or milling operation that would be suitable for restoring the surface 109 of the body 101 to a cylinder conforming shape after a shape twist hardening operation such as carbonitriding. OD grinding leaves traces such as grinding lines and marks. Bore running surface 109 does not carry traces of an OD grinding or any other grinding, milling or lapping operation that would be used to determine its outer diameter 157.

The body 101 also includes a planar surface 113. Planar surface 113 is in the transition region between bore running surface 109 and parallel planar surface 105. The planar surface 113 is adjacent to the parallel planar surface 105 near the end 107 of the parallel planar surface 105, the parallel planar surface 105 being distal to the drive input 103 of the body 101. The planar surface 113 is inclined relative to the axis 151 of the cylinder conforming bore running surface 109. The tilt angle is 40 degrees from the axis 151, which is an angle in the range of 15 degrees to 75 degrees.

The body 101 is hardened by a method of diffusing nitrogen into the metal while maintaining the metal in the ferrite phase. The arrangement of nitrogen atoms within the metal is different from the case where nitrogen is added while the metal is in the austenitic phase. During or after hardening of the shell, the metal is not heated above the critical temperature. Thus, analysis of the distribution of nitrogen within the metal lattice and its structure will show that the component has been case hardened by a method of diffusing nitrogen into the metal while maintaining the metal in the ferrite phase. Analysis can be performed using methods such as X-ray crystallography and scanning electron microscopy.

The lifter 100 is a bucket lifter. Tappet 100 is a high pressure fuel pump actuator, although the same configuration may be used in other tappet applications, such as in a roller lifter. Fig. 7 shows the lifter 100 installed in the engine 150. The engine 150 includes a cylinder head 141 having a bore 143. The tappet 100 is mounted within the bore 143 with its axis 151 coincident with the axis of the bore 143. A smaller bore 145 parallel to the bore 143 and overlapping the bore 143 is also formed in the cylinder head 141. Guide slots may be used in place of bores 145. The anti-rotation guide 115 rides within the bore 145. A spring 171 within bore 145 biases cam follower 131 relative to cam 147. The cam 147 has three lobes. Three lobe and four lobe cams are typical for high pressure fuel pumps. Cams having other numbers of lobes may also be used.

Electronically controlled metering valve 177 is configured to selectively receive low pressure fuel from inlet 179 into pumping chamber 175. As the cam 147 rotates, it drives the tappet 100 upward. The tappet 100 compresses the spring 171 and drives the piston 173 into the pumping chamber 175. The tappet 100 interfaces with the piston 173 via a cross member 121. The cross beam 121 transfers force from the body 101 to the piston 173. In performing this function, the cross beam 121 may be hardened to resist fatigue. The fuel in the pumping chamber 175 is compressed by the piston 173 until it reaches a critical pressure where the check valve 181 opens to release the pressurized fuel to the outlet 183. Once the pressure at the outlet 183 is sufficiently high, a high pressure relief valve 185 may be provided to allow fuel flow back to the pumping chamber 175.

Fig. 8 provides a cross-sectional view of tappet 100 in bore 143. The cross-section corresponds to the tappet cross-section 8-8 identified in fig. 4. The cam follower 131 is removed from this view to provide greater clarity. As shown in this view, bore running surface 109 mates with the wall of bore 143. Diameter 157 may be referred to as the nominal outer diameter of tappet 100. The tappet 100 is a high-pressure fuel pump actuator. Diameter 157 may be any of standard dimensions, including 26mm, 31mm, and 32mm. Thus, diameter 157 may be in the range of 26mm to 32mm. For high pressure fuel pump applications, diameter 157 is typically in the range of about 10mm to about 50 mm. The tappet 100 may alternatively have a larger or smaller diameter.

The diameter 159 of bore 143 is slightly larger than the diameter 157 of bore running surface 109 to provide running clearance. The gap may be in the range of 10 μm to 40 μm. The anti-rotation guide feature 115 extends from the bore 143 into the space of the bore 145. The anti-rotation guide feature 115 cooperates with the wall of the bore 145 to narrowly limit rotation of the tappet 100 within the bore 143. The diameter 161 of bore 145 may be substantially smaller than the diameter 159 of bore 143. Diameter 161 is typically in the range of about 2mm to about 8 mm. Diameter 161 is about 4mm in this example. The cylinder conforming bore running surface 109 has a diameter variance of less than 50 μm. For example, the variance may be 15 μm.

Fig. 9 provides a flow chart of a method 200 that may be used to fabricate lifter 100. The method 200 begins with a strip of sheet metal that is removable from the coil. In act 201, a sheet of metal is subjected to deep drawing to produce a cylinder form. In act 203, the cylinder form is subjected to a series of stamping operations to produce the body 101. These operations may include an act 205 of forming the anti-rotation guide feature 115, an act 207 of forming the parallel planar surface 105, and an act 209 of forming the shaft bore 111. Act 209 includes puncturing and shaving.

Acts 211 to 215 generate and process beam 121 independently of body 101. Act 211 is stamping to form beam 121. Act 213 is neutral hardening. Neutral hardening includes heating the beam 121 above a critical temperature and quenching. Act 215 is tempering, which is a heat treatment method that relieves internal stresses generated during the hardening process.

Act 217 is to install the cross beam 121 within the body 101 and crimp to hold it against the flange 125. The curl forms a dimple 123. The cross beam 121 may be described as a cross web and is mounted within the body 101. Act 219 is Ferritic Nitrocarburizing (FNC), which is a method of case hardening. FNC is a method of adding nitrogen to ferrous metals by diffusion when the metal is below a critical temperature. The critical temperature is the temperature at which the metal begins to transform from the ferrite phase to the austenite phase. The critical temperature is typically about 733 ℃. The FNC process is preferably carried out between 525℃and 625 ℃. The FNC may be a gas FNC method, a salt bath FNC method or a plasma FNC method.

Act 221 is to mount the cam follower 131 to the body 101. The roller 135 is mounted on a bearing 137, and the bearing 137 is mounted on the axial support pin 133. Mounting the cam follower 131 to the body 101 includes fitting the axial support pin 133 through the shaft hole 111. The assembled lifter 100 may be installed in an engine 150, wherein the lifter 100 operates as a fuel pump actuator.

The metal exposed at the bore running surface 109 of the body 101 is substantially the metal present at the outer surface of the metal sheet after deep drawing of act 201, although modified by FNC. The stamping operation 203 has little or no effect on the outer diameter 157. The outer diameter 157 is substantially determined by the deep drawing of act 201, the stamping of act 203, and the FNC of act 219. The outer diameter 157 may be substantially solely determined by the deep drawing of act 201.

The components and features of the present disclosure have been shown and/or described in accordance with certain teachings and examples. Although a particular component or feature, or a broad or narrow representation of such component or feature, has been described in connection with only one embodiment or example, all of the components and features, whether broadly or narrowly represented, may be combined with other components or features as long as such combination is deemed logical by one of ordinary skill in the art.

Claims

1. A tappet comprising:

a body that is a ferrous metal abutment, the body comprising a housing and an interior, the housing comprising a cylinder conforming bore running surface and an anti-rotation guide feature that has been made to protrude outwardly from the bore running surface by stamping; and

a cam follower mounted to the interior of the body;

wherein the outwardly projecting portion and the cylinder conforming bore running surface are located at the same point along the axis of the cylinder to which the cylinder conforming bore running surface conforms;

the cylinder block conforming bore running surface is operable to guide translation of the tappet when installed in a mating bore and limit wobble within the bore;

wherein the outer shell substantially comprises hardening the ferrous metal abutment shell to a first hardness; and, in addition, the processing unit,

wherein the interior has a second hardness of the ferrous metal abutment that is less than the first hardness.

2. A tappet formed by a method comprising:

forming a body from a ferrous metal having a surface, the body comprising a housing and an interior, the surface comprising a cylinder conforming bore running surface on the housing;

stamping the body including the bore running surface to form an anti-rotation guide feature protruding outwardly from the bore running surface;

hardening the outer shell of the body and the anti-rotation guide feature housing to a first hardness by ferritic nitrocarburizing; and

attaching a cam follower to the body on the interior of the body, the interior having a second hardness less than the first hardness;

the cylinder block conforming bore running surface is operable to guide translation of the tappet when installed in a mating bore and limit wobble within the bore.

3. The tappet of claim 2, wherein the method further comprises:

stamping the body at its drive input to form two parallel planar surfaces; and

forming a shaft hole in each of the two flat surfaces;

wherein attaching a cam follower to the body includes mounting an axial support pin for the cam follower through the shaft bore.

4. The tappet of claim 2, wherein the method further comprises:

piercing the body in a stamping operation to form an axial bore in the body;

5. The tappet of claim 2, wherein the method further comprises:

forming a beam of ferrous metal;

hardening the beam by a method comprising heating the beam to a temperature at which the ferrous metal enters an austenitic phase; and

the cross beam is mounted within the body.

6. The tappet of claim 2, wherein forming the body from ferrous metal comprises forming the body from sheet metal by deep drawing.

7. The tappet of any one of claims 1-6, wherein:

the tappet includes a cross beam retained within the body; and is also provided with

The cross-beam is hardened over its full thickness and the body has an unhardened interior.

8. The tappet of any one of claims 1-6, wherein:

the anti-rotation guide feature has a length extending along an axis of the cylinder conforming bore running surface; and is also provided with

The anti-rotation guide feature contacts the cylinder conforming bore running surface along two opposing sides of the anti-rotation guide feature, both of which extend along the length.

9. The tappet of any one of claims 1-6, wherein an interface between the anti-rotation guide feature and the cylinder bore running surface forms a perimeter around the anti-rotation guide feature.

10. The tappet of any one of claims 1-6, wherein the bore running surface is free of any operations that have helped determine its outer diameter and have not been applied to a surface of the anti-rotation guide feature.

11. The tappet of any one of claims 1-6, wherein:

the body further comprising two parallel planar surfaces at its drive input;

forming a shaft hole in each of the two flat surfaces;

an axial support pin for the cam follower is installed through the shaft hole;

the body further comprises two substantially flat additional surfaces;

the two additional surfaces in the transition region between the cylinder conforming bore running surface and the two parallel flat surfaces;

the additional surface is adjacent to the parallel planar surface at the end of the parallel planar surface distal to the drive input end of the body;

the additional surface being inclined relative to the axis of the cylinder conforming bore running surface; and is also provided with

The tilt angle is in the range of 15 degrees to 75 degrees.

12. The tappet of any one of claims 1-6, wherein the tappet is a pump actuator.

13. A method of manufacturing a tappet, the method comprising:

forming a ferrous metal sheet to provide a body, the body comprising a cylinder conforming bore running surface on a housing, and the body comprising an interior;

forming an anti-rotation guide feature protruding outwardly from the cylinder conforming bore running surface by stamping;

a cam follower is mounted at one end of the body on the interior of the body, the interior having a second hardness less than the first hardness.

14. The method of claim 13, wherein the method produces a final outer diameter for the cylinder bore running surface without any grinding, milling or milling affecting the outer diameter of the cylinder bore running surface.

15. The method of claim 13 or 14, wherein stamping further comprises forming an axle bore through which the cam follower is mounted.